Standard platinum resistance thermometers have been used for 50 years as secondary thermometers for determining temperature from standard resistance values in research on temperature standards and in the International Temperature Scale (ITS-90) that require reproducibility and precision of under 0.001° C. FIG. 13 indicates the basic structure of a conventional standard platinum resistance thermometer 1 described in Japan Unexamined Patent Publication No. 2001-343291. Normally, in order to prevent contamination of a highly pure platinum wire 20 that composes a thermosensitive part 2, a gas impermeable protective tube 3 made of quartz, sapphire or the like is used, and purging inside this protective tube 3 is conducted with a mixed gas to which oxygen is intentionally added, for example, argon 85%-oxygen 15%. The small amount of oxygen in this purge gas G is present in order to protect the platinum wire 20 of the thermosensitive part 2 from contamination, and there is no clear standard for the amount, which is determined based on past experience and varies for each manufacturer.
Nonetheless, by adding this oxygen, oxidation of the platinum resistance wire by the oxygen in the purge gas causes the resistance value at temperatures of 300° C. to 500° C. to gradually increase, and variations equivalent to 0.001° C. (1 mK) or more occur. Moreover, when exceeding 600° C., the resistance value variations caused by this oxidation is reversed and the resistance value returns to prior to oxidation. Consequently, if a standard platinum resistance thermometer is used to straddle these two temperature regions, the resistance value of the platinum wire gradually increases and is unstable in the temperature region of 300° C. to 500° C., and then the increased portion of the resistance value is eliminated in the temperature region of 600° C. or more, resulting in the problem that there is no reproducibility in the relationship between the resistance value and the temperature when used again in the temperature region of 300° C. to 500° C.
As a conventional countermeasure, rather than use a standard platinum resistance thermometer in the aforementioned two regions, use was divided between platinum resistance thermometers specific to each temperature region, and maintenance was conducted in accordance to the use conditions (re-annealing and the like) for each thermometer while confirming the characteristics (resistance value drift). These countermeasures were taken because the mechanisms of quantitative resistance value variation by oxidation-reduction of the platinum resistance wire were not understood, and there was a lack of knowledge about changes of platinum resistance wire caused by the oxygen concentration in the purge gas when manufacturing standard platinum resistance thermometers. The above problems are not limited to standard platinum resistance thermometers, but are the same for quasi-standard and industrial ones as well, and the uncertainty caused by oxidation of the platinum has not been discussed in the field of temperature measurements using platinum resistance thermometers and has been virtually ignored in the ITS-90 interpolation formulae in the field of platinum resistance thermometers. The affects of oxidation resistance values cannot be ignored in precision temperature measurements.